1. Chapter 2 Database System Concepts and Architecture
Dr. Bernard Chen Ph.D.
University of Central Arkansas
Fall 2008

2. Outline Data Models, Schemas, and Instances Three-Schema Architecture
Database language and Interfaces
The database system environment
Centralized and client/server architecture

3. Categories of data models
High-level or Conceptual data models:
Provide concept that are close to the way many users perceive data
Low-level or Physical data model:
Provide concepts that describe the details of how data is stored in the computer

4. Conceptual data models
It uses concepts such as entities, attributes and relationships.
Entity represents a real-world object or concept, such as employee or project
Attribute represents some property of interest that further describes an entity, such as employee’s name or salary
Relation among two or more entities represents an association among two or more entitles

5. Example of a Relation

6. Schemas and Database State
In any data model, it is important to distinguish between the description of the data and database itself
The description of the database is called the database schema
A displayed Schema is called a schema diagram

7. University Database

8. Example of a Database Schema

9. Schemas and Database State
The data in the database at a particular moment in time is called a database state
The distinction between database schema and database state is very important
When we define a new database, we specify its database schema only to the DBMS
At this point, the corresponding database state is the empty state with no data
We get the initial state of the database when the database is first loaded
From then on, every time an update operation is applied to the database, we get another database state

10. Schemas and Database State
Valid State: a state that satisfies the structure and constrains specified in the schema.
The database schema changes very infrequently.
The database state changes every time the database is updated
Schema is also called intension.
State is also called extension.

11. Outline Data Models, Schemas, and Instances Three-Schema Architecture
Database language and Interfaces
The database system environment
Centralized and client/server architecture

12. Three-Schema Architecture
Three of four important characteristics of the database approach, listed in Ch.1 are:
Self-describing of a DB (Schema)
Insulation between programs and data
Support of multiple views of the data
Three-Schema Architecture : it was proposed to help achieve and visualize these characteristics

13. Three-Schema Architecture
Defines DBMS schemas at three levels:
Internal schema at the internal level to describe physical storage structures and access paths (e.g indexes).
Conceptual schema at the conceptual level to describe the structure and constraints for the whole database for a community of users.
External schemas at the external level to describe the various user views.

14. The three-schema architecture

15. Outline Data Models, Schemas, and Instances Three-Schema Architecture
Database language and Interfaces
The database system environment
Centralized and client/server architecture

16. DBMS Languages
The first step to create a database through DBMS is to specify conceptual and internal schemas for the database
Data Definition Language (DDL): is used by database designers to define schemas
Data Manipulation Language (DML)
View Definition Language (VDL): is to specify user views
In current DBMS, the preceding types of languages are usually not considered distinct languages

17. DBMS Programming Language Interfaces
Programmer interfaces for embedding DML in a programming languages:
Embedded Approach: e.g embedded SQL (for C, C++, etc.), SQLJ (for Java)
Procedure Call Approach: e.g. JDBC for Java, ODBC for other programming languages
Database Programming Language Approach: e.g. ORACLE has PL/SQL, a programming language based on SQL; language incorporates SQL and its data types as integral components

18. User-Friendly DBMS Interfaces
Menu-based, popular for browsing on the web (URSA)
Forms-based, designed for naïve users
Graphics-based
Natural language: requests in written English
Combinations of the above

19. Outline Data Models, Schemas, and Instances Three-Schema Architecture
Database language and Interfaces
The database system environment
Centralized and client/server architecture

20. DBMS Component Modules
A DBMS is a complex software system
The figure showed in next slide is divide into two halves.
The top half refers to the various users of the database system
The lower half shows the internals of the DBMS responsible for storage of data and processing of transactions

21. Typical DBMS Component Modules

22. Outline Data Models, Schemas, and Instances Three-Schema Architecture
Database language and Interfaces
The database system environment
Centralized and client/server architecture

23. Centralized DBMS Architecture
A centralized DBMS in which all the DBMS functionality, application program execution, and user interface processing were carried out on a single machine

24. A Physical Centralized Architecture

25. Basic Client/Server Architectures
The client/server architecture was developed to deal with computer environment in which a large number of PCs, workstation, file server…
A client in this framework is typically a user machine that provides user interface capabilities and local processing
A server is a system containing both hardware and software that can provide services to the client machines.

26. Logical two-tier client server architecture

27. Two-tier Architecture
This is called two-tire architectures because the software components are distributed over two systems: client and server
The emergence of the Web changed the roles of client and server, leading to the three-tier architecture

28. Three-tier architecture
The intermediate layer or middle layer is sometimes called the application server or Web server
Three-tier Architecture Can Enhance Security:
Database server only accessible via middle tier
Clients cannot directly access database server

29. Three-tier client-server architecture

30. Three-tier architecture
The presentation layer displays information to the user
The business logic layer handles intermediate rules and constrains before data is passed up to the user or down to the DBMS
If the bottom layer is split into two layers (a web server and a database server), then it is a 4-tire architecture (possible to the n-tier)